This lifeform is generally found west of the Continental Divide in North America

Golden Mantled Squirrel (Citellus lateralis) is found widely in the mountain regions of the western United States. This squirrel is found in northern New Mexico, portions of Arizona, and north, including parts of eastern California, Colorado and British Columbia. This small squirrel is typically 8 to 14 inches long. The horizontal white stripe on its body helps identify this species.

Squirrels and woodchucks (Family Sciuridae) are small mammals that are found worldwide. Squirrels, the most common member of the family, live in trees and eat nuts, bark, and pine cones. Chipmunks and ground squirrels are also in this family. There are about 270-280 species in this family.

Rodents (Order rodentia) are a large group of usually small mammals that are found throughout the world. Rodents can be called the gnawing mammals. There are between 1700 and 2300 species, depending upon which authority is used, representing a high percentage of the known mammal species. Duff and Lawson list over 2,100 species.

Mammals (Class Mammalia), together with the birds, are among the youngest of the classes of animals. In species count, mammals number about fifty-one hundred, trailing reptiles (approximately fifty-five hundred), fish (approximately eighteen thousand), and birds (approximately eighty-six hundred).

marsupials
Newborn emerges very underdeveloped and continue to
mature in a pouch on its mother's abdomen (i.e., opossums,
koala, kangaroo)

placental
Embryo develops within the uterus of the female and is
dependent on a placenta for nutrition and waste removal
(i.e., humans, lions, monkeys)

About sixty-five million years ago, the Tertiary era produced thirty-five orders of mammals. Of this number, eighteen have survived to represent Earth's most diversified as well as its most highly developed classification of animals.

Extinction of mammals is fast becoming a serious issue. Duff and Lawson present a list of forty-one extinct species that reached extinction prior to 1800. These forty-one species are not acknowledged in the counts of the various families. Duff and Lawson also present a list of forty-six species including three gazelles, one zebra, one seal, one deer, and one wolf that have probably gone extinct since 1800. These forty-six species are included in the family counts. Science is adding about forty to fifty new species a year to the list. Many of these are the result of divisions of prior species; some are recent discoveries.

Mammals owe their survival to adaptive capabilities that include the ability to exploit whatever sources of food are available to them, as well as their ability to adjust to various climes. Food specialization influenced evolution to such a great extent that the teeth structure can and has been used to provide extensive information on the food needs and various lifestyles of extinct species.

Despite the vast diversity among mammals in terms of size, habitats and adaptations, they share without exception many characteristics such as:

Backboned Animals (Phylum Chordata) are the most advanced group of animals on earth. These animals are characterized by having a spinal cord or backbone. Most members have a clearly defined brain that controls the organism through a spinal cord. Fish, amphibians, reptiles, birds, and mammals are in this phylum.

Currently, some taxonomists believe that the fish should be divided into two groups (sharks and regular fishes) and that there are some other primitive groups in the phylum such as hagfish or lampreys.

Animal Kingdom contains numerous organisms that feed on other animals or plants. Included in the animal kingdom are the lower marine invertebrates such as sponges and corals, the jointed legged animals such as insects and spiders, and the backboned animals such as fish, amphibians, reptiles, birds, and mammals.